Dr. Bartz: When PRPC is converted to PRPSC, it's misfolded. There have to be many different stable energy states for the misfolded protein. I would hypothesize that mink PRPSC, when it interacts with hamster PRPC, it can fold into several different stable PRPSC molecules. So initially you get the mink interacting with hamster, and then you get a strain produced. I think early on in those first few rounds of replication, whatever strain is produced is probably going to be the predominant one because it has a jump start on the rest of them. On this really complicated western blot, we are mixing hyper and drowsy at known ratios, and basically we can mimic these effects. So it really is the ratio of hyper/drowsy produced by interspecies transmission that's causing this sort of effect.

To summarize inter-species transmission, we have PRPSC interacting with the new host PRPC molecule to change it into PRPSC. We think that, in certain instances, multiple strains can be produced. Intra-species transmission results in competition between these strains and eventual emergence of a predominant strain. We think the initial ratio of strains is important and affects this whole passage history. Probably the replication properties of strains is important. We think that drowsy is the predominant strain produced, but hyper replicates so much faster, it has an advantage.

One really important thing I want to point out here is that strain properties can change upon inter-species transmission. Chronic wasting disease doesn't cause disease when you passage it in a hamster, but if you passage CWD into ferrets, and then take that ferret passage tissue, it can cause disease in hamsters. So inter-species transmission can expand the host range. Also, with the hyper and drowsy, the more hamster passages you do, if you back-passage the inoculum into mink, hyper loses pathogenicity for mink quite quickly, where drowsy retains pathogenicity for mink. The important point I want to make is that, when you're assessing inter-species transmission and you do a transmission study and it's negative, you have to be careful in saying it's negative for the strains you looked at. With this example, it's clear you could take hyper TME, inoculate mink, and they don't come down with the disease, so you might assume hamster prions don't cause disease in mink. That strain doesn't. You have to be careful assessing negative transmission results based on what's known about the strain properties.

The last thing I want to talk about is persistence. This would be the case where PRPSC interacts with the host PRPC and you get really slow replication. The replication agent is so slow that the animal dies of old age before clinical signs can occur.

This study is from Rick Race at NIH, transmitting hamster PRPSC into mice. He collected animals post-infection out to 782 days. None of these animals had clinical signs of prion disease, which is consistent with everything we knew about this species barrier. But when he went back and looked for PRP residue in these animals, he couldn't detect hamster PRP residue, but in a few of these animals with very long times post-infection, he could detect mouse PRP residue.

When he did the second passage, into either hamsters or mice, clinical signs appeared in the second passage. The point is that first inter-species transmission may not cause clinical signs, but you still can get replication to agent that subsequently, when you passage it into the same host species, results in clinical signs of the disease.

In the cell-free conversion studies, hamster PRPSC could not convert mouse PRPSC. Every sort of assay has limitations. The cell-free conversion said it couldnít replicate. It could, but it was so slow and so long that the assay could not detect them.

I think persistence is very important. If you have inter-species transmission occurring and it doesn't cause clinical disease, and if you take the tissue and keep feeding it to that same host species, youíre going to get amplification and potentially emergence of the disease.

Is PRPSC shed in the environment? I have no idea. Terry can talk about that. Does PRPSC survive in the environment? The studies on deer PRPSC have not been done, but if deer PRPSC behaves like any other PRPSC, yes it can survive in the environment. Can PRPSC reach a new host species? I don't know. If they share common pastures, it's a possibility. Can PRPSC get to the central nervous system? Clearly, cattle are susceptible to oral infection, so that's yes. Can deer PRPSC convert cattle PRPC to the host PRPSC? Self-reconversion experiments would say yes, but very inefficiently. But really, the gold standard is the transmission studies, and there are two of these ongoing right now. One is at the USDA at Ames, and this is intra-cerebral inoculation. They are susceptible to IC infections. This means that once the agent reaches the brain, it can cause disease, but obviously in the field, that's not the natural route. Beth Williams is doing some oral infection studies, but I'm not sure of the status of those.

Dr. Thornsberry: So what youíre saying is that, inter-cerebrally, we can get CWD/PRPSC conversion, but that has not occurred, to anyone's knowledge, in the natural route.

Dr. Bartz: Right. IC inoculation is used because it has a short incubation period. It only tells us that replication can occur once the agent reaches the brain.

Dr. Thornsberry: Let's hypothesize that I had some cattle on the eastern slope and they were in the same pasture with elk with CWD. If a cow had been exposed to the PRP Scrapie and it did develop disease four years later, would that look like BSE? Would there be a way to determine if it came from CWD?

Dr. Bartz: The IC studies in cattle indicate it does not look like BSE. The clinical signs of the IC/CWD cattle are more like downer cattle, and not aggressive. As far as finding the source of a bovine TSE, the gold standard is the lesion profile study where you take cattle tissue and inoculate it into mice with appropriate controls, wait until the mice come down, and do the lesion profiling.

Dr. Thornsberry: There were two cases in Japan, but they indicated that tissue was not classical BSE as seen in Europe. Have you heard anything about that?

Dr. Bartz: This is based on differences on migration and the glycoform ratio of PrPSc.

Dr. Detwiler: Canada based that question because the herd that that animal came from was in Saskatchewan, in an area with CWD. That was one of the questions they faced right off the bat: is this BSE or is this some kind of transmission from CWD-infected elk in the area? Not only the histological lesions were classic BSE lesions, but clinically it's very difficult because if you miss the other behavioral changes, which this owner did. It was someone who had been a catfish farmer. He missed the early signs. The animal presented to slaughter as a down animal, non-responsive. Clinically it looked like just a down cow, but they did send that on to the United Kingdom and they did do some comparison glycoform patterns. Those havenít been validated, but at least on preliminary work, it looked like classical BSE.

The Japanese case was a 23-month-old which was born in October. Their scientists say the western blot pattern looked different. The most recent case, which was a 21-month-old, looked more like classic BSE. The Italian cases were older animals, 15 and 16 years of age. But is it methodology? Is it really standard? That has to be sorted out before too much can be said.

Dr. Bartz: Glycoform ratio is dependent on very technical matters, what antibodies you use, what detection system you use. Those have to be standardized before you can start comparing from one lab to another.

Dr. Detwiler: The Japanese used a western blot they'd developed in their lab. It can't be compared across laboratories.

Dr. Bartz: That's problematic.

Accomplished this day, Wednesday, December 11, 2003, Denver, Colorado

The roundtable presentations and discussionswere recorded. A transcript will be made availableto the Academy of Veterinary Consultants, theAmerican Association of Bovine Practitioners, andthe Colleges of Veterinary Medicine throughout theUnited States and Canada. A condensed versiontranslated for the livestock industry will be madeavailable to educate livestock producers aboutprion related diseases.